1. Field of the Invention
The present invention relates to an analytical apparatus and in particular a mass spectrometry system including an electron impact ionizer.
2. Description of Related Art
Mass spectrometry (MS) is a commonly used analytical technique for determining the mass of particles. MS can also be used to determine the elemental composition of a sample or molecule by analyzing its constituent parts, and to provide an insight into the chemical structures of molecules, for example complex hydrocarbon chains. A mass spectrometer determines the mass of a particle by measuring its mass-to-charge ratio. This method requires the particles to be charged, and a mass spectrometer therefore operates by ionizing samples in an ion source to generate charged molecules and/or molecular fragments and then measuring the mass-to-charge ratios of these ions.
Uncharged particles (neutrals) cannot be accelerated by an electric field. It is therefore necessary that all particles to be analyzed by mass spectrometry are ionized. A typical ionization technique is electron ionization (EI), also referred to as electron impact ionization, in which a source of gas phase neutral atoms or molecules is bombarded by electrons. The electrons are normally generated through thermionic emission in which an electric current is passed through a wire filament to heat the wire causing the release of energetic electrons. The electrons are then accelerated towards the ion source using a potential difference between the filament and the ion source.
EI is a routinely used technique usually intended for the analysis of low-mass, volatile, thermally stable organic compounds. EI is normally performed at an electron energy value of 70 eV as this presents high ionization efficiency and an analytical means of standardization across different MS instruments offering this ionization technique. However, at an electron energy of 70 eV the energy transferred from the accelerated electrons to the sample molecules during ionization impact is sufficient to break bonds within the analyte molecule causing it to ‘fragment’ into several smaller ions. Ordinarily this is desirable, since the energy deposition causing molecular fragmentation is reproducibly standardized such that the pattern of fragment ions, the ‘mass spectrum’ of a given analyte, is sufficiently similar on different instruments to yield an analytical fingerprint for the analyte. The level of fragmentation is such that, for many chemical classes of analytes, the original molecule (or ‘molecular ion’) often cannot be seen or is very small. For this reason EI is known as a ‘hard’ ionization technique.
For mixtures of analytes, a hyphenating analytical technique such as gas chromatography (GC) is often interfaced to the mass spectrometer, enabling highly complex mixtures of analytes to be separated in time and sequentially admitted to the ion source. But even with analytical hyphenation, the complexity of the sample may be overwhelming and cause many superimposed mass spectra to be generated which cannot be unraveled and collectively defy analytical discrimination. Therefore it is often desirable to reduce the degree of fragmentation by reducing the energy of the electron ionization. However, if the electron energy is lowered by reducing the electron acceleration voltage a marked decrease in ion production is experienced in part due to a decrease in the concentration of electrons in the ion source as the electrical field is insufficient to accelerate significant numbers of electrons away from the filament in a concentrated path, and in part to a reduced ionization efficiency at electron energies below 70 eV. The latter effect is shown in FIG. 1, which charts ionization probability vs. electron energy for some example molecules. A peak is displayed at around 70 eV and the sensitivity below 70 eV decreases sharply until a level is reached, typically at around 15 eV, where the results are usually not analytically useful.
By increasing the current of the electron emission filament, the population of electrons generated will increase and the ion flux may also increase, leading to some improvement in sensitivity at lowered electron energies. However, at large filament currents the high densities of electrons close to the filament causes Coulombic repulsion (called Space Charge Limited Emission, also known as Child-Langmuir Law in the case of planar geometry), where the repulsive forces between the high density electrons proximal to the filament itself prevent further electrons from being released. This results in an electron flux plateau. Furthermore, in regions of high electron density around the filament, the electrons which have been released are also repelled from one another. This results in a broadening of the electron beam which can reduce the accuracy with which the electrons are focused into the ion source and therefore the level of ionization. This issue is amplified when the electrons have lower kinetic energy due to a lower applied potential difference, as their momentum in the direction of the ion source is decreased. As such, increased filament current may only provide a limited improvement in ionization efficiency.
Chemical ionization is a known ‘soft’ ionization technique. Chemical ionization requires the use of large quantities of a reagent gas such as methane and the ionization energy is dependent on the reagent gas used. Therefore the ionization energy is not easily adjustable. Standardization of spectra can also be difficult with this method due to a shortage of libraries to search.
A number of alternative soft ionization techniques have been applied to GC/MS measurements. These include resonance-enhanced multi photon ionization (REMPI) and the more universal single photon ionization (SPI). These soft ionization methods cause little or no fragmentation of the molecular ion which have been applied to sources in GC/MS instruments. Another soft ionization technique uses the cooling of the molecules in a supersonic molecular beam (SMB). A SMB is formed by the expansion of a gas through a pinhole into a vacuum chamber resulting in the cooling of the internal vibrational degrees of freedom. SMB is used as an interface between a GC and an MS and combined with electron impact ionization lead to enhanced molecular ion signals and can therefore be regarded as a soft ionization method.
Such ‘soft’ ionization techniques provide soft ionization only and cannot be utilized to also provide harder ionization if such is required. US2009/0218482 describes a system which provides both hard and soft ionization using electron pulses to create hard electron ionization of the analyte molecules and photon pulses to provide soft photo ionization. These two techniques are implemented simultaneously with the electron ionization being repeatedly switched ‘on’ and ‘off’ in a pulsed manner to switch between the soft and hard ionizations. However, the hardware requirements for such a system are significant with both electron and photon generation means being required together with the associated delivery and focusing set up for each technique. The cost of such a dual system is therefore prohibitive and the amount and size of equipment required to implement both ionization techniques significantly increase the space required for such a system.
It is therefore desirable to provide an improved ionization apparatus and method for the ionization of an analyte sample which addresses the above described problems and/or which offers improvements generally.